Science & Speed Camera Enforcement

Scientific Basis for the Implementation of Speed Camera Enforcement in South Africa

In an urban 60 km/h speed limit environment ... the relative risk of being involved in a casualty crash ... was found to approximately double for each 5 km/h increase in free travelling speed.

There is a strong statistical relationship between speed and road safety. When the mean speed of traffic is reduced, the number of accidents and the severity of injuries will almost always go down. When the mean speed of traffic increases, the number of accidents and the severity of injuries will usually increase.

1.1 Studies found:

– There is a strong statistical relationship between speed and crash rates.

– Speeding is the number one road safety problem in most countries around the world

1.2 Speed Cameras:

Changing driver behaviour is the key

The best way to change the behaviour of motorists who speed is through a combination of education and enforcement.

But are cameras the best way to effect this change? The answer is ‘Yes’.

The study monitored the number and severity of casualties over time and compared crashes to areas that did not have heightened speed camera activity.

In areas where there were very high levels of speeding enforcement, the fatality risk was reduced by 41%.

Conversely, where there were relatively low levels of speed camera fines, in following months the fatality risk increased by 44%.

All types of speed enforcement examined have led to at least some positive impact on either casualty crash frequency, crash severity or driver behaviour. In most cases, this effect has been significant.”

3.1 Scientific basis for the strategic directions of the safety camera programs in South Africa

Signed, fixed speed cameras are a cost-effective means of addressing specific areas where crashes occur and the use of unsigned, mobile cameras reduce speeding across the entire road network rather than just where a camera is known to be located.

Cars involved in casualty crashes (cases) were generally travelling faster than cars that were not involved in a crash (controls):

68 per cent of crash involved cars were exceeding 60 km/h compared to 42 per cent of those not involved in a crash (Table 2).

The difference was even greater at higher speeds: 14 per cent of crash involved cars were travelling faster than 80 km/h in a 60 km/h speed zone compared to less than 1 per cent of those not involved in a crash.

The crash-involved cars were almost 10 times more likely to have been travelling faster than 70 km/h than were the non-crash-involved cars (29% vs 3%).

Table 4.2
Percentage of Vehicles
Travelling Above the Given Speeds

Speed (km/h)

Per cent above speed

Cases

Controls

50

94.7

88.7

60

67.5

42.1

65

47.7

12.9

70

29.1

3.0

75

19.2

0.7

80

13.9

0.5

85

8.6

0.0

90

6.0

0.0

95

3.3

0.0

100

2.6

0.0

4.1 Travelling Speed and the Relative Risk of Involvement in a Casualty Crash

In this section we present the risk of involvement in a casualty crash at specified speeds relative to the risk for drivers travelling at 60 km/h. The speeds of the cases (the crash-involved drivers) and the controls (those not involved in a crash) are grouped in 5 km/h intervals as shown in Table 3.

Table 3
Travelling Speed and the Risk of Involvement in a Casualty Crash
Relative to Travelling at 60 km/h in a 60 km/h Speed Limit Zone

Nominal
Speed

Speed
Range

No. of
Cases

No. of
Controls

Relative
Risk

35

33-37

0

4

0

40

38-42

1

5

1.41

45

43-47

4

30

0.94

50

48-52

5

57

0.62

55

53-57

19

133

1.01

60

58-62

29

205

1.00

65

63-67

36

127

2.00

70

68-72

20

34

4.16

75

73-77

9

6

10.60

80

78-82

9

2

31.81

85

83-87

8

1

56.55

-

88+

11

0

infinite

Total

151

604

Travelling Speed

60 km/h

70 km/h

Cases

29

20

Controls

205

34

The relative risk (R.R.) of involvement in a casualty crash at a travelling speed of 70 km/h compared to 60 km/h is calculated as follows:

That is, a driver travelling at 70 km/h in a 60 km/h speed zone has a risk of being involved in a casualty crash that is more than four times greater than that of a driver travelling at the speed limit. The figure of 4.16 is actually the relative odds of involvement in a casualty crash.

It is possible to calculate the range of values that probably includes the 'real' relative risk (Gart, 1962) and the limits of this range are shown in Table 3. For the above example, the 95% confidence limits are 2.12 and 8.17. This means that the 'real' relative risk has a 95% probability of being within the range from 2.12 to 8.17. If, as here, the interval between the confidence limits does not include 1.00 then it can be said that the risk of involvement in a casualty crash at the specified travelling speed (here it is 70 km/h) is statistically significantly different from the risk at a travelling speed of 60 km/h.

A statistically significant difference is not necessarily large enough to be of practical importance. The results listed in Table 3, however, show that even a travelling speed of 65 km/h doubles the risk of involvement in a casualty crash. An increase in risk of that magnitude is clearly of practical importance.

None of the travelling speeds below 60 km/h was shown to be associated with a risk of involvement in a casualty crash that was statistically significantly different from the risk at 60 km/h. There was some indication that the risk decreased somewhat to 50 km/h and then increased to greater than one at 40 km/h but the confidence intervals show that this trend could well have arisen purely from random variation.

Above 60 km/h, however, there is a steady increase in risk of involvement in a casualty crash with increasing travelling speed such that the risk approximately doubles with each 5 km/h increase in travelling speed.

The information in Table 3 is presented graphically in Figure 3. The representation of the speed data in 5 km/h intervals provides a clear picture of the change in risk by speed.

Figure 3
Travelling Speed and the Risk of Involvement in a Casualty Crash
Relative to Travelling at 60 km/h in a 60 km/h Speed Limit Zone

Note: Relative risk at 60 km/h set at 1.00.
95 per cent confidence intervals are shown by the vertical lines.

4.2 Free Travelling Speed Crash Types

Each crash involving a free travelling speed case vehicle was classified into one of 11 crash types. In crashes with multiple case vehicles, the crash type was classified separately for each vehicle. The average travelling speed of the case vehicles and the associated controls in each category was also calculated. The results are shown in Table 4.

Table 4
Crash Type and Average Travelling Speed

Crash Type

Number
of Cases

Per cent
of Cases

Average Case
Speed (km/h)

Average Control
Speed (km/h)

Oncoming vehicle turned right across path

55

36.4

68.9

59.0

Vehicle entering from left turned right across path

23

15.2

63.0

58.6

Loss of control followed by collision

14

9.3

82.6

63.3

Rear end collision with vehicle in front

14

9.3

63.5

60.4

Hit pedestrian or bicyclist

12

7.9

62.8

61.6

Vehicle crossing in front from right to left

9

6.0

65.2

56.4

Vehicle doing U-turn in front

8

5.3

65.1

60.6

Vehicle crossing in front from left to right

7

4.6

62.7

60.3

Hit by an out of control vehicle

7

4.6

66.4

65.0

Vehicle on right turned right into path

1

0.7

66.0

61.3

Side swiped vehicle travelling in the same direction

1

0.7

92.0

58.0

Total

151

100.0

67.6

59.9

The most common crash types in the sample were an oncoming vehicle turning right across the path of the free travelling speed vehicle (36%) and a vehicle turning right from the side street on the left of the free travelling speed vehicle (15%). These two categories accounted for over half of all the crash types.

Disregarding the last two categories in Table 4 because of the single cases, the crash types associated with the highest free travelling speeds were:

losing control of the vehicle followed by a collision (average speed = 83 km/h); and

having an oncoming vehicle turn right across the path of the free travelling speed vehicle (average speed = 69 km/h).

4.3 Hypothetical Crash Outcomes at Reduced Travelling Speed

Additional information about the effects of travelling speed was obtained by calculating what the hypothetical outcome for the vehicles and those people injured in the case crashes would have been if the case vehicle had been travelling at a different speed.

4.4 Injuries Sustained in the Crashes

In each of the 148 crashes in this study, at least one person was injured sufficiently to require transport to a hospital. In total, 237 persons received an injury from these crashes.
Table 5 shows the outcome, mostly in terms of the level of treatment, of the injured persons in the crashes investigated (based on police report information). Those cases listed under "Transported by ambulance" were known to have been transported to hospital but it was not listed on the police report whether they were treated in the casualty department and discharged or admitted to the hospital for longer term treatment.

Table 5
Injury Outcome

Injury Outcome

Number

Per cent

Injured but not treated

3

1.3

Treated by private doctor

8

3.4

Transported by ambulance

25

10.5

Treated at hospital

133

56.1

Admitted to hospital

62

26.2

Fatality

6

2.5

Total

237

100.0

4.5 Hypothetical Outcomes at Reduced Travelling Speeds

For each crash, five hypothetical speed reduction scenarios were applied to the free travelling speed of the case vehicle (or to multiple case vehicles if appropriate).

The results are expressed in terms of four factors:

an estimated reduction in the number of crashes and

persons injured due solely to those crashes not happening; and,

in those crashes that would still have occurred, the reduction in the change in velocity (delta V) and

the crash energy experienced by the injured parties (see Table 7).

Table 7
Hypothetical Outcomes at Reduced Travelling Speeds

Hypothetical Situation

% Reduction
in number
of Crashes

% Reduction
in number
of Persons
Injured*

% Reduction
in average
Delta V**

% Reduction
in average
Crash
Energy**

10 km/h speed reduction

41.5

34.6

25.5

38.7

5 km/h speed reduction

15.0

13.1

16.1

23.6

Limit 60 km/h with total compliance

28.6

30.4

11.8

21.7

Limit 50 km/h with compliance as at present

32.7

26.6

24.9

37.5

Limit 50 km/h on local streets only with compliance as at present

6.1

4.2

2.8

4.7

Note that the percentage reduction in the number of persons injured is an underestimate since most of the crashes that would still occur under a hypothetical lower travelling speed situation would have occurred at a lower speed than was actually the case and would therefore have had a lower chance of causing injury. For example, some of the cases under the hypothetical scenarios would have had an impact speed of only a few km/h so, even though the crash would still have taken place, it is almost certain that no injury would have resulted. Also, some of the drivers who lost control of the case vehicle would probably not have done so under a hypothetical lower travelling speed situation and in some cases the other vehicle may not have misjudged the case vehicle's speed and created a crash situation.

A uniform 10 km/h reduction in the travelling speeds of the case vehicles offered the greatest reduction in the number of crashes (42%) and persons injured (35%) and also offered the greatest reduction in crash energy experienced by injured parties in crashes that would still have taken place (39%).

The 5 km/h reduction scenario had much less effect on the elimination of crashes (15%) but still reduced the average crash energy level experienced by the injured parties in those crashes that still would have occurred by 24 per cent.

An alternative estimate of the effect of the elimination of speeding (limit 60 km/h with total compliance) can be derived from the data in Table 3. The Table shows that 93 (62%) of the case vehicles were speeding (speed greater than the 58-62 km/h band). If none of the case vehicles had been speeding (ie. their relative risk was reduced to 1.0), fewer casualty crashes would have occurred. Working back from the relative risk figures, we would expect that 50 per cent of the crashes in the 65 km/h band might have been avoided (or been reduced from a casualty crash to one not requiring ambulance transport), rising to 98 per cent of the 85 km/h crashes, and virtually all of the crashes involving vehicles above 87 km/h.

By applying this method to all of the cases exceeding 62 km/h (Table 8) it can be seen that the elimination of speeding would be expected to reduce free travelling speed casualty crashes by about 46 per cent.

Table 8
The Effect of the Elimination of Speeding
on Free Travelling Speed Casualty Crashes